If you are using the STM32Cube FW_F7 package from ST, you might want to refer to the emWin example project found under:
STM32Cube_FW_F7_V1.8.0\Projects\STM32746G-Discovery\Demonstration\STemWin\

In the subfolder of this directory (\Core\Src) you will find a file named k_bsp.c. In this file the touch input from the LCD gets passed to emWin using the function GUI_TOUCH_StoreStateEx().
Use this files as reference or if possible use it as is.

Beside of that you can also download our example project for this device from our webiste: